Messages in a network such as the Internet can be distinguished into unicast messages, broadcast messages and multicast messages. The difference between these types of messages involves addressing of these messages. A unicast message is addressed at a single terminal of the network, a broadcast message is addressed at all terminals of the network and a multicast message is addressed at a group of a plurality of the terminals of the network.
A multicast message typically includes a group address as a destination address, rather than the addresses of individual terminals. In principle such a multicast message could be broadcast to all terminals of the network, each terminal inspecting the messages for the presence of a group address to determine whether the message is relevant for the terminal. In large networks, however, multicast routers are used to restrict transmission of multicast messages to relevant terminals. This means that the multicast routers must have information about the terminals that need to receive multicast messages with specific group addresses. When a host in the network indicates that it has to receive multicast messages with a specific group address, a multicast router records that messages with that group address must be routed to the terminal to which the host is connected. The multicast router can record this for a plurality of terminals, so that it will route copies of the multicast message to the plurality of terminals.
In a co-pending patent application, which is assigned to the same assignee, and unpublished at the priority date of the present application, it has been described how different subscribers of a telephone network can share each others' subscriber lines to provide high peak bandwidth, using inverse multiplexing of message traffic. For this purpose, an inverse multiplexer is provided (typically in a telephone exchange) that distributes message traffic for one subscriber over the subscriber lines of a plurality of subscribers. At each subscriber site an inverse demultiplexing device is provided. The inverse demultiplexing devices of different subscribers are cross-connected to communicate inversely multiplexed messages, and each inverse demultiplexing device is arranged to reassemble a message stream for its subscriber by means of inverse demultiplexing. In this way decentralized inverse demultiplexing is used to reassemble the streams for different terminals. The destination of the messages determines which of the demultiplexing devices will reassemble the message stream.
This inverse demultiplexing scheme is preferably implemented so that it is transparent for the remainder of the network, including multicast routers. This means that this scheme can readily be combined with all network functions, including multicast transmission.
A set of inversely multiplexed lines may be used downstream from a multicast router. If a single terminal is located downstream of the inversely multiplexed lines this requires no additional measures: the stream of multicast messages is inversely multiplexed and demultiplexed on its way to the terminal if the terminal has subscribed to the multicast stream. If more than one potential subscribers are located downstream of the inversely multiplexed lines two multicast routers are preferably used, one upstream of the inversely multiplexed lines and one downstream, so that the upstream multicast router will send a stream of multicast messages to the downstream multicast router via the inverse multiplexed lines and the downstream multicast router will send copies of the stream to different subscribers terminals (or further multicast routers).
However, this has the disadvantage that a central multicast router is needed downstream of the inversely multiplexed lines. This downstream multicast router may form a bottleneck for message traffic.